1. Field of the Invention
The present invention is broadly concerned with compositions for dissolving scales (and particularly metal sulfate scales) and methods of using the same. More particularly, the scale dissolver compositions of the invention comprise an aqueous dispersion which includes a salt of a chelating agent, a carbonate, a base, and a component selected from the group consisting of organophosphorus compounds, carboxylic acids, and mixtures thereof. In use, the dissolver compositions are contacted with the target scale, preferably after first heating the composition. The compositions are particularly useful for removing calcium sulfate and barium sulfate scales from gas and oil well equipment, boilers, and pipes.
2. Description of the Prior Art
Many waters contain alkaline earth metal cations such as barium, strontium, calcium, and magnesium, and anions such as sulfate, bicarbonate, carbonate, phosphate, and fluoride. When combinations of these anions and cations are present in concentrations which exceed the solubility product of the various species which may potentially be formed, precipitates form until the respective solubility products are no longer exceeded. For example, when concentrations of barium and sulfate ions exceed the solubility product of barium sulfate, a solid phase of barium sulfate will form as a precipitate. The formation of solubility products are affected by several factors including pH, temperature, pressure, and the presence of a seed crystal. When these solubility products form, they precipitate on the surface of water-carrying or water-containing systems and form adherent deposits or scales which cause serious problems. For example, during gas and oil production, scale plugging of surface and subsurface equipment, tubings, and perforations often lead to severe productivity declines and difficult operating conditions. Of the sulfate scales, barium sulfate is particularly troublesome because of its extreme insolubility.
Previous methods have been attempted to dissolve sulfate scales. For example, U.S. Pat. No. 5,183,112 to Paul et al. discloses a method for dissolving calcium sulfate wherein a scale-removing solvent comprising a chelating agent and the anions of a monocarboxylic acid, oxalates, or thiosulfates is injected into a well followed by sparging of the solvent with air or an inert gas. However, sparging with air or gas is a difficult and messy process, and is quite messy.
Prior art attempts at dissolving barium sulfate include utilizing boiling sulfuric acid. This is impractical and dangerous for dissolving downhole barium sulfate. Another method for removing barium sulfate is to fracture (xe2x80x9cfracxe2x80x9d) it with extremely high pressure. However, this process is quite costly in that it requires special equipment and extra manpower, and results in lengthy well downtimes. Another prior art process for removing barium sulfate is to perforate the scale. This is also not very effective. In some instances, when scales present too much of a problem, the well is simply abandoned and a new well is drilled when scales present too much of a problem. Obviously, this is a very costly solution.
The present invention overcomes the prior art problems by providing an effective, inexpensive scale dissolver that is particularly effective at dissolving sulfate scales such as alkaline earth metal sulfate scales. Furthermore, the dissolver compositions of the invention are relatively easy to use.
The scale dissolver compositions comprise an aqueous dispersion which includes a salt of a chelating agent, a carbonate, a base, and a component selected from the group consisting of organophosphorus compounds, carboxylic acids, and mixtures thereof The chelating agent is preferably a salt of EDTA (or mixture of EDTA salts) such as EDTA tetrasodium salt (sold under the tradename VERSENE 100), trisodium EDTA, or the edetates. The chelating agent should be present in: the composition at a level of from about 10-70% by weight, and preferably from about 30-50% by weight, based upon the total weight of the composition taken as 100% by weight.
Suitable carbonates for use in the instant invention include all alkali metal carbonates and bicarbonates, with sodium bicarbonate being particularly-preferred. Preferably, the carbonate is present in the composition at a level of from about 0.1-10% by weight, and more preferably from about 2-6% by weight, based upon the total weight of the composition taken as 100% by weight
While most bases are suitable for use with the dissolver compositions, it is preferred that the base comprise an OH group. Preferred bases include ammonium hydroxide (preferably at a solution concentration of from about 28-30%) and pure NH3 gas. The base should be present in the composition at a level of from about 0.1-10% by weight, and more preferably from about 2-6% by weight, based upon the total weight of the composition taken as 100% by weight.
When the scale to be dissolved comprises barium sulfate, it is preferred that an organophosphorus compound, such as an organophosphonic acid, be utilized in the dissolver compositions of the invention. A particularly preferred organophosphonic acid is aminotrismethylidine diphosphonic acid sold under the tradenames PHOS-2, DEQUEST 2000, and UNIHIB 305. The organophosphorus compound is preferably present in the composition at a level of from about 0.1-10% by weight, and more preferably from about 2-6% by weight, based upon the total weight of the composition taken as 100% by weight.
When the scale to be dissolved comprises calcium sulfate, it ispreferred that a carboxylic acid (preferably one having from 2-6 carbon atoms) be included in the dissolver composition. A particularly preferred carboxylic acid for use in the instant invention is hydroxyacetic acid. The carboxylic acid should be present in the composition at a level of from about 0.1-10% by weight, and more preferably from about 2-6% by weight, based upon the total weight of the composition taken as 100% by weight.
The compositions of the invention are formed by preparing a mixture comprising the chelating agent salt, the carbonate, and the component selected from the group consisting of organophosphorus compounds, carboxylic acids, and mixtures thereof. The desired base is then added to this mixture approximately 10-15 minutes after the mixture is formed. It is important to wait this amount of time in order to prevent the carbonate from falling out of solution. The final mixture should have a pH of from about 7-8.5, preferably from about 7.2-8.0, and more preferably about 7.5. If necessary, the pH should be adjusted to this range by utilizing small quantities of the base, the carbonate, and/or organophosphorus compound as necessary to reach the desired pH.
In use, the dissolver compositions are simply contacted with the scale to be dissolved or removed. The quantity of dissolver composition required is generally from about 0.5-5 gallons of composition, and preferably from about 1-3 gallons of compositon, per 2.65 lbs of scale. It is preferred, that the composition be heated to a temperature of from about 100-170xc2x0 F., and more preferably from about 120-150xc2x0 F., prior to the contacting step in order to more efficiently remove/dissolve the scale. Advantageously, after about 0.5-3 hours at least about 75% of the scale, and preferably at least about 100% of the scale, is dissolved or removed from the contaminated surface.
Those skilled in the art will appreciate that the instant compositions are particularly useful for removing downhole scale in oil and gas wells thus providing an effective alternative to the prior art methods. When treating oil and gas wells or pipelines with the dissolver compositions, the composition is simply injected into the well or pipeline so that it contacts the target scale as described above. Again, it is preferred that the composition first be heated to the above-described temperatures prior to injection. After injection, hot oil or hot wateris preferably flushed into the well or through the pipeline.